| THE EAR. 699 
aspect of the optic stalk, forming what is termed the chorioidal fissure (Fig. 508). Through 
this fissure the mesoderm passes inwards between the lens and the retina to form the vitreous 
body, while the arteria centralis retinze also becomes enclosed in it and so gains its future 
position in the centre of the optic nerve. The arteria 
centralis is prolonged forwards from the porus opticus 
through the vitreous body, as a cone of branches, as 
far as the back of the lens. By the fifth or sixth month 
all these branches have disappeared except one, the 
arteria hyaloidea, which persists until the last month 
of foetal life, when it also atrophies, leaving only the 
canalis hyaloideus to indicate its position. 
The lens rudiment, at first in contact with the ecto- 
derm, from which it is derived, soon becomes separated Fic. 508.—Opric Cup anp LENS VIEWED 
“Optic stalk 
» Arteria 
centralis 
Chorioidal 
fissure 
~Lens 
from it by mesoderm, and then consists of a rounded FROM BEHIND AND Below, to show 
vesicle lined by epithelium. The epithelium which A a ee ts SU aE a 
enclosure of the arteria centralis re- 
lines the anterior part of the vesicle remains as a tine (from model by Ziegler). 
single layer of cells—the anterior lens epithelium of ; 
the adult. The cells lining the posterior part of the vesicle become elongated into lens 
fibres, and by the forward growth of these the cavity of the vesicle is obliterated. This 
elongation into lens fibres is greatest at the centre of the lens, while near the equator 
the fibres are shorter, and here the gradual transition between the anterior epithelium 
and the lens fibres is seen (Fig. 506). The lens becomes enveloped in a vascular tunic, 
which receives its vessels from the arteria centralis retinze and from the vessels of the iris. 
The front part of this tunic forms the membrana pupillaris, and this, like the rest of the 
tunic, disappears before birth. 
The hollow stalk of the optic cup becomes solid by the thickening of its walls and the 
obliteration of its cavity, and, acquiring nerve fibres, becomes the optic nerve. These nerve 
fibres are mostly centripetal, and are derived from the nerve-cells of the retina ; but afew 
are centrifugal and have their origin in the brain. The further development of the retina 
resembles, in certain respects, that of the spinal cord. Some of the epithelial cells of the 
retinal layer become elongated and branched spongioblasts, and are developed into the 
sustentacular fibres of Miller and their limiting membranes; while others, neuroblasts, 
give rise to the nervous elements. The rods and cones make their appearance in the 
human embr yo about the end of the fifth month (Falchi), but not until after birth in the 
cat and rabbit (M. Schultze). 
The condensed mesoderm surrounding the optic cup becomes the sclera and chorioid. 
In the portion of the mesoderm which lies in front of the lens a cleft-like fissure appears, 
and divides it into a thick anterior and a thin posterior layer. The former becomes the 
substantia propria of the cornea; the latter, the stroma of the iris and anterior part of the 
vascular tunic of the lens. The fissure represents the future anterior chamber, and its 
lining cells form the layer of endothelium on the back of the cornea and front of the 
iris. 
The eyelids arise as two integumentary folds above and below the cornea, each being 
covered on both its surfaces by the ectoderm. By the third month the folds meet and 
unite with each other at their edges, the eyelids being only permanently opened shortly 
before birth ; in many animals they are not opened until after birth. The ectoderm 
forms the epithelium of the conjunctiva and the stratified epithelium of the cornea. It 
is also invaginated at the lid margins to form the hair follicles and the lining cells of the 
Meibomian glands and glands of Moll, and, at the fornix conjunctivee, to form the lining 
of the alveoli and ducts of the lachrymal gland. 
The nasal duct, lachrymal sac, and canals represent the remains of the furrow which 
extends from the inner angle of the eye to the nasal cavity between the superior maxillary 
and lateral nasal processes (p. 38). It is at first filled by a solid rod of cells, which 
becomes hollowed out to form the duct and canals. 
> 
THE EAR. 
The ear or organ of hearing (organon auditus, Fig. 509) consists of three por- 
tions—external, middle, and internal, the last constituting its essential part, as 
within it are distributed the peripheral terminations of the auditory nerve. 
